It is known that lower olefins can be prepared from carbon monoxide (CO) and hydrogen (H2) over metal catalysts, e.g. iron or cobalt catalysts. Iron oxides are usually used as catalyst precursors. Such catalysts are described, for example, in U.S. Pat. No. 4,544,674, U.S. Pat. No. 5,100,856, U.S. Pat. No. 5,118,715, U.S. Pat. No. 5,248,701, US 2004/0127582 A1, H. P. Withers et al., Ind. Eng. Chem. Res. 1990, 29, pages 1807 to 1814, and M. E. Dry et al., Stud. Surf. Sci. Catal., Vol. 152, 2004, pages 533 to 600.
This reaction is also referred to as the Fischer-Tropsch synthesis.
Conventional processes for the Fischer-Tropsch synthesis produce hydrocarbons having a wide product distribution.
In principle, this range of the product distribution can be characterized by the Anderson-Schulz-Flory distribution; cf.: M. Janardanarao, Ind. Eng. Chem. Res. 1990, 29, pages 1735-53.
It is likewise known that the composition of the hydrocarbons formed in the Fischer-Tropsch process can be strongly influenced by the choice of catalysts used, the types of reactor and the reaction conditions.
For example, it is known that the product distribution can be shifted in the direction of lower olefins by use of high temperatures in the presence of modified iron catalysts: B. Büssemeier et al., Hydrocarbon Processing, November 1976, pages 105 to 112.
The main problem here is the formation of large amounts of undesirable methane (CH4).
In addition, the iron oxides required as starting material for the catalyst are difficult to reduce.
DE 28 22 656 A1 (Inst. Fr. du Petrole) discloses a Fischer-Tropsch process in which the catalyst is obtained by precipitation of a metal-organic iron and/or cobalt and/or nickel aggregate on an inorganic support. The precipitation of the aggregate on the support is effected by impregnating the support with a solution of the aggregate. C2-C4-Olefins (“lower olefins”) and only small amounts of methane are said to be formed selectively in this process. The main disadvantage of these catalysts is that the active catalyst constituents can be volatile under the reaction conditions, which results in a loss of metal, and that they are toxic.
DE 29 19 921 A1 (Vielstich et al.) describes a further Fischer-Tropsch process in which catalysts comprising polycrystalline iron whiskers as substantial catalyst component are used. These iron whiskers are obtained by thermal decomposition of iron pentacarbonyl in a magnetic field. The iron whiskers are preferably used as pellets. According to the teachings of this DE document, polycrystalline whiskers are fine iron threads having microscopically small single crystal regions (page 5, 3rd paragraph). The shape of the thread-like primary particles results from growth in the magnetic field. The threads have a length of, for example, from 0.06 to 1 mm.
The two figures in “Fachberichte für Oberflächentechnik”, July/August 1970, page 146, show scanning electron micrographs of such a carbonyl iron powder having thread-like primary particles.
“Fachberichte für Oberflächentechnik”, July/August 1970, pages 145 to 150, also describes these iron whiskers as metal hairs which result from crystal growth of the metal in thread form, unlike normal crystal growth (page 145, 2nd paragraph). In the polycrystalline iron whiskers, the ratio of length to diameter is, for example, ≧10.
Such polycrystalline iron whiskers are also described in H. G. F. Wilsdorf et al., Z. Metallkde. 69 (11), 1978, pages 701 to 705.
DE 25 07 647 A1 (Kölbel et al.) describes the use of catalysts comprising manganese and optionally iron for preparing hydrocarbons and oxygen-comprising compounds from CO and H2.
U.S. Pat. No. 2,417,164 (Standard Oil Comp.) relates to processes for synthesizing liquid hydrocarbons from CO and H2 in the presence of metal catalysts, including carbonyl iron powder.
WO 07/060,186 A1 (BASF AG) teaches processes for preparing olefins from synthesis gas using Fischer-Tropsch catalysts in a reaction column.
WO 09/013,174 A2 (BASF SE) relates to a process for preparing short-chain, gaseous olefins by reacting carbon monoxide with hydrogen in the presence of an iron-comprising heterogeneous catalyst, where carbonyl iron powder having spherical primary particles is used as catalyst.
Promoters in iron catalysts for Fischer-Tropsch syntheses are described, for example, in the abovementioned WO 09/013,174 A2 and in M. Janardanarao, Ind. Eng. Chem. Res. 1990, 29, pages 1735 to 1753, and C. D. Frohning et al. in “Chemierohstoffe aus Kohle”, 1977, pages 219 to 299.
As suitable promoters, the catalyst can comprise, for example, one or more of the elements potassium, vanadium, copper, nickel, cobalt, manganese, chromium, zinc, silver, gold, calcium, sodium, lithium, cesium, platinum, palladium, ruthenium, sulfur, in each case in elemental form or in ionic form.
EP patent application no. 08164085.6 (BASF SE) of Sep. 10, 2008 describes an integrated process in which pure carbonyl iron powder (CIP) is prepared by decomposition of pure iron pentacarbonyl (IPC) in a plant A, in which carbon monoxide (CO) liberated in the decomposition of the IPC is used for preparing further CIP from iron in plant A or is fed to an associated plant B for preparing synthesis gas or is fed to an associated plant C for preparing hydrocarbons from synthesis gas and the CIP prepared in plant A is used as catalyst or catalyst component in an associated plant C for preparing hydrocarbons from synthesis gas from plant B.
Two parallel European patent applications having the same filing date (all BASF SE) relate to particular iron-comprising heterogeneous catalysts and their use in processes for preparing olefins by reacting carbon monoxide with hydrogen.